PROJECT SUMMARY/ABSTRACT Age-related changes in cellular functions are influenced by the local and systemic environment in which a cell resides. This has been demonstrated unequivocally by heterochronic studies in which aged cells are introduced or exposed to a more youthful environment, resulting in enhanced cellular function reminiscent of more youthful counterparts. Conversely, young cells exposed to an aged environment adopt a more aged phenotype. These findings all point to circulating factors that may profoundly influence cellular aging. In the studies of this proposal, we seek to extend our own findings that emerged from studies of heterochronic parabiotic mice implicating active Wnt signaling, mediated by one or more systemic factors, as a driver of muscle stem cell (MuSC) aging. It was later shown that a component of the complement cascade, C1q, increased in the blood with age, is capable of activating Wnt signaling in cells, and is responsible for age- related impairment in muscle regeneration, although the cellular targets were not identified. C1q has subsequently been shown to function independent of the classic cascade in multiple physiologic processes and to drive aging in a variety of tissues. For the studies of this proposal, we will seek to explore the mechanisms by which C1q inhibits muscle regeneration by exploring the cellular targets and signaling cascades associated with C1q activity. Toward these goals, the Specific Aims of this proposal are (1) to study the mechanism by which C1q promotes the age-related decline in muscle regenerative potential (and specifically whether MuSCs are the direct target); (2) to test whether C1q promotes cellular aging by inducing TGF? signaling (based on evidence we and others have obtained in previous studies of this pathway being downstream of Wnt signaling and a mediator of MuSC aging); and (3) to test whether C1q promotes age-related fibrogenesis in skeletal muscle by acting directly on fibroadipogenic progenitors (FAPs) (to examine the cellular mechanism of the demonstrated effects of C1q in promoting muscle fibrosis). Together, these studies will expand our understanding of the effects and mechanisms of this systemic mediator of cellular aging and the exciting new findings of non-canonical mechanisms of complement components and their newly discovered roles in tissue aging.